This invention pertains to reciprocating pumps, generally, such as compressed air-operated, double-diaphragm pumps, having a pump-operating, main spool valve or shuttle valve, and in particular to such a non-lubricated, air-actuated, pump-operating, shuttle valve arrangement, in a reciprocating pump.
Compressed air-operated, diaphragm pumps are susceptible to stalling; stalling occurs when the pump-operating main spool or shuttle valve becomes halted at the midpoint of its reciprocating motion in its bore. In this circumstance, the pump is prevented from restarting without repositioning the stalled main spool or shuttle valve. This condition can occur when the compressed air source is interrupted with the pump running, or when there is a low air supply pressure. In prior art pump designs which incorporate shuttle valves or shafts which use O-rings in dynamic sealing conditions, the O-rings can become set, during periods of non-use, or the shaft can become frozen in position so as to become "stalled". Manufacturers of these prior art designs, even though they claim lubrication free configurations, must use oil or grease of some kind to lessen such stalling. Restarting of these designs, if possible, would require significantly higher than normal operating inlet air supply pressures.
Prior art, air-operated, diaphragm pumps, typically, are unable to operate at low inlet air supply pressures, and they are known to be susceptible of unreliable operation at extremely low pump discharge flow rates. Too, the prior art, air-operated, diaphragm pumps have an inability to operate smoothly and reliably during constant, start-stop, duty cycles.
Most current versions of air-operated, diaphragm pumps utilize a lubricated pilot spool or lubricated main shaft (which also serves to pilot the main spool or shuttle valve), which requires O-rings to seal in a dynamic condition. In order to qualify for non-stall operation, the O-rings need to be lubricated by oil mist or grease pack to be able to slide without binding at low air supply pressures.
Polytetrafluorethylene-encapsulated O-rings were tried, to eliminate the need for lubrication in the aforesaid prior art pumps, but these were unsuccessful.
At least one, current, "non-stall" air valve manufacturer, which claims its pilot spool with O-rings requires no outside lubrication, requires the use of an in-line lubricator on the air supply for other than intermittent operation. For continuous operation, light oil injection is recommended.
One other, current, "non-stall" air valve manufacturer utilizes an unbalanced main spool, or shuttle valve, to allow a greater force to bias the main spool or shuttle valve in one direction to prevent centering thereof, and resultantly, stalling. This design comprises a grease pack to allow the sliding of close tolerance components to occur.
Further, current versions of such air valves utilize two spring-loaded actuators (i.e., popper valves) to shift the main spool or shuttle valve. Even though these versions were lubrication free, they had a tendency to hang-up or fail to shift completely, on low air supply pressures, or on constant start-stop operation. Since the spring in a spring-loaded popper valve allows the valve to remain open only momentarily, a low inlet air pressure condition results in an insufficient volume of air to reach the main spool or shuttle valve. Further, in these low pressure applications, this incomplete main spool shift can result in "centering" or air valve "hang-up". This low inlet air pressure condition is aggravated by designed "controlled leakage" to exhaust which reduces further the volume of air which is available to cause main spool or shuttle valve shifting. The necessity to overcome the spring force to open a spring-loaded actuator is also disadvantageous in a low supply air pressure condition.
Known, prior art embodiments of such air valves also have an inordinate number of discrete parts and components, and/or complicated arrangements and assemblies which make any routine maintenance and servicing very troublesome and expensive.